class Item < ActiveRecord::Base
has_many :basket_items

  def withdraw(num)
    adjust_balance_and_save(-num)
  end

  def deposit(num)
    adjust_balance_and_save(num)
  end

  private
  def adjust_balance_and_save(num)
  self.quantity += num
  save!
  end

  def validate
  errors.add(:quantity, "is negative") if quantity < 0
  end
end